Sea-ing red: Marine heat wave days nearly quadruple, 4.8 million fish dead

The seas around Korea are running a fever. Marine heat waves have occurred 3.7 times as often over the past decade as the long-term average. The prolonged heat is already taking a mounting toll on fish farms and marine ecosystems.

The average sea surface temperature in waters around Korea rose 0.7 degrees Celsius (1.26 degrees Fahrenheit) above the 1991 to 2020 average according to an analysis released by the Korea Meteorological Administration (KMA) on Wednesday.

Over the past decade, marine heat waves occurred for an annual average of 83.3 days. The figure translates to the seas around Korea experiencing heat-wave conditions for nearly three months each year. The figure was 3.7 times the long-term average of 22.6 days.

A marine heat wave occurs when the daily average sea surface temperature remains above the 90th percentile of the long-term distribution for at least five consecutive days.

The average intensity of marine heat waves also rose to 2.1 degrees Celsius from the long-term average of 1.9 degrees. The East Sea saw the most pronounced change with marine heat waves lasting an average of 96.1 days a year.

“Over the past decade, the global average sea surface temperature rose 0.3 degrees Celsius compared to the long-term average while waters around Korea warmed by 0.7 degrees,” the KMA said. “This shows that although ocean warming is occurring worldwide it has been relatively more pronounced in waters around Korea.”

Hotter ocean could trigger much bigger problems.

“Unusually high sea surface temperatures around the world show that the climate crisis in our oceans is becoming a reality,” KMA Administrator Lee Mi-seon said. “The ocean interacts closely with the atmosphere and can be linked to extreme weather events such as heat waves and heavy rainfall. That can affect people across many aspects of their daily lives.”

The rise in marine heat waves is also taking a severe toll on marine ecosystems and the fishing industry.

Persistently high water temperatures have caused mass deaths at fish farms this summer. More than 4.8 million farmed fish died due to high water temperatures from May 15 through Tuesday, according to the Ministry of Oceans and Fisheries.

In South Gyeongsang, fish farms also made an emergency release of 5.78 million farmed fish to the ocean to minimize losses from high water temperatures.

Widespread "slumping events" were found among soft corals around Mun Island and Beom Island off Seogwipo, Jeju Island, according to underwater studies by the Paran Ocean Citizen Science Center. The phenomenon causes soft corals to collapse or deflate and can eventually kill them as they lose internal fluid pressure.

Marine heat waves are expected to become even more frequent and intense if current climate change trends continue.

The KMA projects that by the end of the century, waters around Korea could experience marine heat waves for 304 days a year even under a low greenhouse gas emissions scenario. If greenhouse gas emissions remain at moderate levels, marine heat waves could occur for as many as 320 days a year — leaving Korea’s surrounding seas unusually warm for most of the year.

Prolonged high water temperatures could also sharply reduce the period suitable for producing gim (dried seaweed) and threaten the survival of the seaweed farming industry.

“If water temperatures continue to rise due to warming the damage will worsen with problems such as seaweed seedlings rotting,” Hong Sun-kee, a professor at Mokpo National University and head of its Institution for Marine and Island Cultures, said. “The seaweed farming industry itself could face a crisis.”

BY CHON KWON-PIL [[email protected]]